Differential/comparative analysis of two gas samples is often used to perform precise and accurate molecular detection, e.g. in isotopic ratio measurements when sample under test must be compared with a reference standard preferably at the same time. Typically, in such applications two strategies are applied: (i) laser beam is divided into two beams and two photodetectors are used (one for each beam interacting with different sample), or (ii) laser beam is directed through a single gas cell which is filled with a first sample and subsequently is detected using a single photodiode. The process is then repeated with a second gas sample. This process measures an alternated signal originating from both gases. In the first case, the measurement can be affected by the difference in specifications (responsivity, linearity etc.) of both detectors as well as by the uncorrelated drift of any parameters of the two optical branches (thermal drifts affecting the optical system dimensions, difference in optical interference effects observed in each optical arm, electronic or thermal drifts of the electronics etc.). In the second case, switching of the gas mixture inside the gas cell requires a complex gas handling, the cell evacuation process is slow, and any nonlinearity of the detector can also affect measurement results. Due to unavoidable drifts in the system the slow gas exchange process will reduce reliability and long-term stability of the setup. Therefore improved differential/comparative analysis systems are desirable.